1. Field of the Invention
This present invention is related in general to high efficiency power amplifier systems and methods. More particularly, the invention is directed to systems and methods for power amplifiers exhibiting wide instantaneous signal bandwidth.
2. Description of the Prior Art and Related Background Information
The surge in wireless data traffic has continued over the past two decades and there is no evidence of a change in this trend. Given the propagation properties of the electromagnetic energy, for optimum coverage, carrier frequencies at lower frequency bands (i.e. from a fraction of a Giga Hertz to a few Giga Hertz) are preferred. However, the commercial competition for access to bandwidth resources is also fierce which limits the available bandwidth for each service. The limited available bandwidth allocated to cellular communication has resulted in a gradual but steady “systems bandwidth efficiency” improvement. By implication, more sophisticated modulation schemes may be devised to improve the system throughput and its figure of merit such as bit/Hertz/s. The byproduct of these developments are that, throughout the system air interface, from transmitter to receiver end, the carrier signals have to preserve precise amplitude and phase modulation, and therefore, less distortion is tolerated. At the same time, both amplitude and phase (frequency) of the carrier now bear information which leads to a modulated carrier with relatively high amplitude variation. Hence, most digitally modulated carrier signals used in modern telecommunication systems have an amplitude envelope showing a large peak to average ratio. In such systems, to preserve modulation accuracy and prevent transmitter spurious emissions, the amplifying device has to maintain high linearity by having sufficient headroom for the signal peaks, albeit producing a modest average output power and therefore leading to low DC to RF conversion efficiency.
Even from the early days of AM broadcasting and in more recent complex transmission systems such as satellite communications, cable TV, and more recently cellular telephony, the carrier amplifiers have been mostly used in conjunction with some means of linearization to achieve the required performance. Feedback, and in RF frequency bands, feedforward systems have been widely used. Analog predistortion has been also used since the early days of satellite communication where frequency division multiple access (“FDMA”) systems were employed for sharing transponder bandwidth. In recent years, with the advent of digital signal processing (“DSP”), digital predistortion has become the preferred choice.
Nonetheless, despite the significant advances in linearization of RF power amplifiers, such techniques typically come at the expense of amplifier sub-system efficiency. Therefore, it is desirable to have additional improvement to achieve linearity and improve efficiency in RF amplifier subsystems.
It is well known that amplifiers exhibit high efficiency when they are operated at their maximum output capability and are actually driven into saturation. The Doherty amplifier is one of the popular techniques for efficiency enhancement and is based on the above principle for efficiency enhancements. In a classic Doherty pair comprising of a main amplifier 10 and peaking amplifier 12 shown in FIG. 1, the main amplifier 10 is biased at class-AB and reaches its saturation state some 6 dB below the total rated output power of the pair and, therefore, shows high efficiency at back off. As signal envelope increases, the peaking amplifier 12 biased at class-C turns on and pulls the main amplifier load down while increasing the main amplifier load increases gradually. This mode of operation widely known as load-modulation allows the main amplifier's load to decrease at higher power levels, and in effect, has a linearizing consequence. Hence the Doherty amplifier efficiency enhancement results by extending the saturation region to a much wider range of output power while maintaining a reasonable degree of linearity.
The Doherty pair is normally used as the final stage in an amplifier chain and is preceded by several lower power linear driver amplifier stages. The basic amplifier architecture referenced in prior art is shown in FIG. 2. In this basic architecture, the output of driver stage 14 is divided in two halves by signal splitter 16 which drives the main amplifier 10 and peaking amplifier 12 and hence must be sized sufficiently to generate the required drive level. Given the fact that at back off region of input signal excursion, the energy delivered to the peaking amplifier 12 is not actually used, and the driver is a linear stage, it seems an area for enhancing Doherty power amplifier (“PA”) efficiency may be to move the driver stage 14 after the signal splitter 16. By doing so, two smaller driver stages can be used and the driver for the peaking amplifier 12 can be also a class-C stage in embodiments disclosed herein and hence saving driver energy waste. FIG. 3 illustrates a modified Doherty configuration which is based on U.S. Pat. No. 7,362,170 B2 to Louis. However, the approach outlined in the above referenced patent does not result to an optimum solution in two respects: cost and also performance over wide bandwidth. The embodiments described in the following section may disclose a new approach leading to a cost effective and ultra broad band performance.
Accordingly, a need exists to improve the performance of power amplifiers.